The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are lower magnetic shield 10 on which is seed layer 11. Antiferromagnetic (AFM) layer 12 is on seed layer 11. Its purpose is to act as a pinning agent for a magnetically pinned layer. The latter is typically a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer 14 between two antiparallel ferromagnetic layers 13 (AP2) and 15 (AP1).
Next is a copper spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. Capping layer 18 lies atop free layer 17. When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 10-20%.
Earlier GMR devices were designed to measure the resistance of the free layer for current flowing parallel to its two surfaces. However, as the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP) have also emerged. CPP GMR heads are considered to be promising candidates for the over 100 Gb/in2 recording density domain (see references 1-3 below).
A routine search of the prior art was performed with the following references of interest being found:
U.S. Pat. No. 6,683,762 (Li et al) disclose Ta/NiFe as an example of a seed layer and IrMn as the possible composition of the AFM layer. Note that, although a Ta/NiCr seed layer is mentioned in the specification, this appears to have been a typographical error since only Ta/NiFe seeds are specified in the claims.
In U.S. Pat. No. 6,574,079, Sun et al. discuss Ta and NiCr alloys for the seed layer. However, they disclose Ta only, NiCr only or a Ta—NiCr alloy; which behaves differently from a Ta/NiCr two layer structure.
U.S. Patent Publication 2002/0191356, Hasegawa et al. teach an underlayer of Ta, seed layer with NiCr together with an AFM layer of IrMn. However, this is for a CIP GMR structure where the Ta/NiCr seed layer is grown on alumina. In the present invention, the seed layer is grown on NiFe bottom shield and is for CPP application. Additionally the present invention teaches a Ta thickness range of around 3-10 A, a thickness range not claimed in 2002/0191356.
In U.S. Pat. No. 6,636,389, Gill shows a deposited NOL layer between two free layers which is a different use of a NOL from that disclosed by the present invention. In U.S. Patent Publication 2004/0004261, Takahashi et al. mention an NOL-GMR but provide no details. What is discussed is a half-metal oxide layer as the AP1 or free layer in a CPP GMR. It has no bearing on the present invention.
In U.S. Patent Publication 2002/0048127, Fukuzawa et al. discloses Ta as the seed layer and IrMn as the AFM layer. Cu is not mentioned as the base metal, but plasma etch and oxidation to form the NOL layer are discussed in great detail. This invention is directed to specular NOL layer formation in AP1 or CIP GMR applications. The NOL structure and plasma etch/oxidation conditions are totally different from those of the present invention.